Week 04 · Intermediate Track

Week 4 — Standard Library

The Python standard library has batteries included. Learn the most useful modules: datetime for time, pathlib for files, collections for advanced data structures, and itertools for efficient looping.

Intermediate Track2 min readFree lesson

Level: Intermediate | Prerequisite: Week 3 (Module/Package)

Learning Goals

Handle date and time with datetime
Random values and sampling with random
File and folder paths with pathlib
Counter, defaultdict of collections

The standard library can be imported directly without additional installation.

1. datetime

from datetime import datetime, date, timedelta

now = datetime.now()
print(now)
print(now.strftime("%Y-%m-%d %H:%M"))

today = date.today()
birthday = date(2000, 5, 9)
days_lived = (today - birthday).days

next_week = now + timedelta(days=7)

2. random

import random

print(random.random())                  # 0.0 ~ 1.0
print(random.randint(1, 100))           #Integers 1 ~ 100
print(random.choice(["Scissors","Rock","Paper"]))
print(random.sample(range(1, 46), 6))   #Lotto
random.shuffle(my_list)                 #In-place shuffling

3. pathlib

Handle paths more cleanly than strings.

from pathlib import Path

p = Path("data") / "report.txt"        # data/report.txt
p.parent.mkdir(parents=True, exist_ok=True)
p.write_text("hello", encoding="utf-8")
print(p.read_text(encoding="utf-8"))

print(p.exists(), p.suffix, p.name, p.stem)

#List of files in the folder
for f in Path(".").glob("*.py"):
    print(f.name, f.stat().st_size)

4. collections

from collections import Counter, defaultdict, deque

#Counter — Frequency count, one line
text = "to be or not to be"
print(Counter(text.split()))    # {'to': 2, 'be': 2, ...}
print(Counter(text.split()).most_common(2))

#defaultdict — Default value if key is missing
groups = defaultdict(list)
for name, age in [("A", 20), ("B", 25), ("C", 20)]:
    groups[age].append(name)
print(dict(groups))

#deque — Double-ended queue
dq = deque([1, 2, 3])
dq.appendleft(0)
dq.append(4)
print(dq)

Key Examples (examples/)

File	Content
`01_datetime.py`	Date/Time
`02_random.py`	Random
`03_pathlib.py`	Path
`04_collections.py`	Counter, defaultdict

⚠️ Common Mistakes

datetime vs date — datetime includes time, date only date.
Don't know random.seed() — Set a seed like random.seed(42) to reproduce the same results.
Adding path strings — Path("data") / "x" is safer than "data" + "/" + "x".

❓ FAQ

Q1. Difference between time and datetime? A. time is low-level (timestamp), datetime is high-level (object). Usually, datetime is enough.

Q2. Which to use, os.path or pathlib? A. For new code, pathlib is recommended. More OOP-like and readable.

📝 Exercises (exercises/)

exercise1.md — D-Day calculator
exercise2.md — Print sizes of .txt files in a folder
exercise3.md — Top 5 word frequencies (Counter)

Next Week

Week 5. Exception Handling

Example files

The runnable files used in this lesson. Copy them into your own project and experiment.

examples/01_datetime.pyPython

from datetime import datetime, date, timedelta

now = datetime.now()
print(now)
print(now.strftime("%Y-%m-%d %H:%M:%S"))
print(now.strftime("%Y year %m month %d day %A"))

today = date.today()
birthday = date(2000, 5, 9)
delta = today - birthday
print(f"{delta.days} days since birth")

next_week = now + timedelta(days=7)
print(f"Next week: {next_week.strftime('%Y-%m-%d')")

#Parse string to datetime
parsed = datetime.strptime("2026-05-09 14:30", "%Y-%m-%d %H:%M")
print(parsed)

examples/02_random.pyPython

import random

print(random.random())
print(random.randint(1, 100))
print(random.choice(["Scissors", "Rock", "Paper"]))
print(random.sample(range(1, 46), 6))

#Seed fixed → Reproducible
random.seed(42)
print([random.randint(1, 10) for _ in range(5)])

#Shuffle list (in place)
nums = [1, 2, 3, 4, 5]
random.shuffle(nums)
print(nums)

#Weighted selection
result = random.choices(["A", "B", "C"], weights=[5, 3, 1], k=10)
print(result)

examples/03_pathlib.pyPython

from pathlib import Path

#Path Combination
p = Path("data") / "report" / "summary.txt"
print(p)

#Create Parent Folder
p.parent.mkdir(parents=True, exist_ok=True)

#Write/Read
p.write_text("Hello
Python!", encoding="utf-8")
print(p.read_text(encoding="utf-8"))

#Information
print("Exists:", p.exists())
print("Name:", p.name)
print("Extension:", p.suffix)
print("Name (without extension):", p.stem)
print("Size:", p.stat().st_size)

#Traverse Folder
for f in Path(".").glob("*.py"):
    print(f.name)

examples/04_collections.pyPython

from collections import Counter, defaultdict, deque

# Counter
text = "the quick brown fox jumps over the lazy dog the quick"
words = text.split()
c = Counter(words)
print(c)
print(c.most_common(3))

#defaultdict — Automatically created if the key does not exist
groups = defaultdict(list)
for name, age in [("A", 20), ("B", 25), ("C", 20), ("D", 25)]:
    groups[age].append(name)
print(dict(groups))

#deque — Fast addition/removal from both ends
dq = deque([1, 2, 3])
dq.appendleft(0)
dq.append(4)
print(dq)
dq.popleft()
dq.pop()
print(dq)

Assignment

Goal

Takes the target date as input and outputs the remaining days.

Requirements

Format: YYYY-MM-DD
Calculate the difference with today
If past, D+N; if future, D-N

Input/Output Example

Target Date (YYYY-MM-DD): 2026-12-25
Today: 2026-05-09
D-230 until Christmas

Task 2. .txt File Sizes in Folder

Goal

Outputs all .txt files and their sizes (bytes) in a given folder.

Requirements

Use pathlib.Path.glob
Sort by size in descending order

Input/Output Example

Folder: ./data
report.txt           1,234 bytes
notes.txt              456 bytes
todo.txt                12 bytes
Total 3 files, 1,702 bytes

Task 3. Top 5 Word Frequencies (Counter)

Goal

Prints the top 5 most frequently appearing words in a long text.

Requirements

Use collections.Counter
Case-insensitive
Remove periods, commas, etc.

Hint

import re
words = re.findall(r"\w+", text.lower())

Show the reference solution

Try the assignment yourself first — comparing your attempt with a working solution is where most of the learning happens.

solution1.pyPython

from datetime import date

target = input("Target Date (YYYY-MM-DD):").strip()
y, m, d = map(int, target.split("-"))
target_date = date(y, m, d)
today = date.today()

delta = (target_date - today).days
print(f"Today: {today}")
if delta > 0:
    print(f"D-{delta} until the target date")
elif delta < 0:
    print(f"D+{-delta} since the target date")
else:
    print("D-DAY!")

solution2.pyPython

from pathlib import Path

folder = Path(input("Folder path:").strip() or ".")
files = sorted(folder.glob("*.txt"), key=lambda p: -p.stat().st_size)

total = 0
for f in files:
    size = f.stat().st_size
    total += size
    print(f"{f.name:20} {size:>8,} bytes")
print(f"Total {len(files)} files, {total:,} bytes")

solution3.pyPython

from collections import Counter
import re

text = """
The quick brown fox jumps over the lazy dog. The dog is sleeping.
The fox runs quickly. Quickly! The fox ran over the dog.
The dog wakes up.
"""

words = re.findall(r"\w+", text.lower())
counter = Counter(words)

print("Top 5:")
for word, count in counter.most_common(5):
    print(f"{word}: {count} times")

Learn this lesson with a mentor

Work through the Python Series track on your own, or join a guided cohort at our Pitampura campus and online.

Enroll now Track overview